Each participant will be enrolled in the trial for approximately 72-76 weeks. Each participant will receive assigned treatment at Weeks 0 and 4 in Part I. At Week 16, the dosage of treatment the patient is assigned to may be adjusted based on the Psoriasis Area and Severity Index (PASI) 75 response (responder vs non-responder). Participants will receive study medication once every 12 weeks during Part 2 (Weeks 16 to 52); no participants will receive placebo in Part 2. Part 3 is an observational period and each subject will continue to be monitored on a monthly basis through Week 72. Subjects will not receive any study medication during Part 3.